Prochiral and pseudoasymmetric centers: Implications of recent definitions

New definitions of prochiral and of pseudoasymmetric elements have been proposed by Prelog and Helmchen [Helv. Chim. Acta55, 2581 (1972)]. The revised definition of prochiral centers would fail to identify many centers bearing like ligands that can be distinguished experimentally. The new definition of the pseudoasymmetric center would cause some that were previously so designated to be considered chiral. Whenever this occurs, concepts like retention and inversion of configuration would no longer have their customary meaning. Concomitant changes in the sequence subrules would not always provide reliable information about the possibilities of stereoisomerism. A modification of the rules is outlined which would avoid this, but secure similar benefits.